A South Korean Marine displayed signs of being mentally unstable long before he killed four colleagues in a shooting spree apparently fuelled by alcohol, a military investigator said Tuesday. The 19-year-old corporal identified only as Kim fired at least a dozen rounds Monday at the Marine Corps barracks on Gangwha island west of Seoul, killing four Marines and injuring one other. Kim himself was hospitalised after being injured by a hand grenade blast in an apparent suicide attempt. His injuries were believed to be serious but not life threatening. \"I feel so miserable. I just want to die,\" he told investigators in a written message from his hospital bed, according to Yonhap news agency. News reports said his vocal cords were damaged in the blast. \"There should be no more beating or bullying,\" he reportedly added, complaining that neither his superiors or his juniors respected his rank. Chief investigator Captain Kwon Young-Jae told an earlier briefing that a military psychological test conducted earlier during Kim\'s service had found him to be unstable and to have difficulty coping with military life. \"His superiors had found him displaying odd behaviour... we are of the view that the accident was largely due to issues in his personality and mental state,\" Kwon said. But the military said the test, conducted about one year ago, did not indicate Kim was a danger to others. Kwon said that just before the incident a barracks mate had seen Kim, smelling of liquor, staggering with a red face and declaring he would kill a colleague -- who was later shot dead. \"He apparently had been drinking, which under the rules is prohibited,\" Kwon said, adding several bottles of liquor were found nearby. Kim also left a message containing complaints about his life and a letter that appeared to be a will, Kwon said, refusing to give details. A memo that military investigators found in Kim\'s locker read: \"I hate myself. I\'m a troublemaker. So many (people) are trying to change me,\" according to Yonhap news agency. Kim likely took the K-2 rifle, bullets and a hand grenade from a weapons storage room that was mistakenly left open and unattended, Kwon said. The elite Marine Corps is charged with guarding frontline islands in the Yellow Sea near the tense disputed border with the North. But Monday\'s incident -- the third in six years -- raised questions about standards of discipline in the South\'s largely conscript 650,000-strong military. Able-bodied South Korean men must undergo at least two years\' military service and some complain of abuse and harassment. Eight soldiers were killed and two seriously injured in 2005 when a soldier threw a grenade and sprayed bullets over sleeping colleagues at a frontline guard post north of Seoul. The attacker alleged senior colleagues had bullied him. In 2008 an army private struggling to adapt to military life threw a grenade at sleeping colleagues, injuring five. Marines and other military units have been on high border alert for months, after the South accused the North of torpedoing a warship in March 2010 and killing 46 sailors. The North denied the warship attack but shelled a South Korean border island last November, killing two civilians and two Marines.